At some point in the processing of semiconductor wafers, it is common to need to etch one dielectric layer selectively relative to an underlying dielectric layer. This is typically accommodated by utilizing chemically different dielectric layers, such as an oxide and a nitride, and utilizing a wet acid or other chemistry which enables etching of the overlying material relative to the underlying material. One drawback with such techniques is that typically the two different dielectrics do not have the same thermal coefficient of expansion. This can create undesired stress in the resulting structure where remaining adjoined layers of an oxide and a nitride exist.
Silicon dioxide can be provided atop a wafer in both doped and undoped forms. Example dopants include phosphorus and boron, which can be utilized to produce phosphosilicate glass (PSG), borosilicate glass (BSG) and borophosphosilicate glass (BPSG). Such dopants can improve step coverage, diffusion barrier properties, stress reduction, and improve flow characteristics enabling dielectric flow at lower temperatures. Additionally, doped and undoped silicon dioxide has very similar thermal expansion coefficients.
It would be desirable to develop a wet etching chemistry that would enable etching of doped silicon dioxide selectively relative to undoped silicon dioxide.